Electric power conversion apparatus having frequency fixing unit to control beat phenomenon

Abstract

An electric power conversion apparatus includes a converter 1 for rectifying ac electric power, a series-connected set, of capacitors 2 A and 2 B, connected to the dc side of the converter, an inverter 3 for, by setting one end of the series-connected capacitor set to a high-potential level, the other end of the set to a low-potential level, and a connection point between the capacitors to a medium-potential level, selecting any one of the levels and outputting three-phase ac, an inverter control unit 4 for controlling the inverter 3 , and a voltage measurement device 8 for measuring a capacitor voltage Vdc as a voltage between both ends of the series-connected set, of the capacitors 2 A and 2 B; and the inverter control unit 4 further includes a beatless control unit 21 for controlling a modulation factor γ in response to the capacitor voltage Vdc, and a frequency fixing unit 22 for fixing to a command value the frequency of an ac voltage outputted from the inverter 3.

1. An electric power conversion apparatus comprising:
 a converter for rectifying ac electric power; 
 a capacitor connected to a dc side of the converter; 
 an inverter for converting into ac electric power dc electric power stored in the capacitor; 
 an inverter control unit for controlling the inverter; and 
 a voltage measurement device for measuring a capacitor voltage which is a voltage of the capacitors; 
 wherein the inverter control unit comprising a beatless control unit for obtaining a compensated modulation factor by calculating a modulation factor to be inversely proportional to the capacitor voltage, and 
 a frequency fixing unit for switching to output the modulation factor or the compensated modulation factor every cycle of a frequency of an ac voltage outputted from the inverted in response to a frequency at which a beat phenomenon occurs, so as to fix to a command value a frequency of the ac voltage outputted from the inverted. 
 
     
     
       2. An electric power conversion apparatus as recited in  claim 1 , wherein the frequency fixing unit outputs the compensated modulation factor obtained by the beatless control unit during a part of one cycle period in a predetermined frequency range in which the beat phenomenon occurs, and outputs the modulation factor in a predetermined frequency range in which the beat phenomenon does not occur. 
     
     
       3. An electric power conversion apparatus as recited in  claim 1 , wherein the frequency fixing unit outputs the modulation factor, for each phase, during predetermined periods in every cycle of a frequency of an ac voltage outputted from the inverter,
 each of the predetermined periods includes a timing at when ac voltage of the phase changes and a line voltage between the phase and another phase changes to a zero value or from a zero value, and 
 the timing is such timing either that the line voltage, changing at the timing from a nonzero value of one polarity to a zero value, changes to a nonzero value of the other polarity as first change from the zero value after the timing, or that the line voltage, changing at the timing from a zero value to a nonzero value of one polarity, changes from a nonzero value of the other polarity to a zero value as last change to the zero value before the timing. 
 
     
     
       4. An electric power conversion apparatus as recited in  claim 1 , wherein the capacitor consists of a series-connected set of capacitors. 
     
     
       5. An electric power conversion apparatus as recited in  claim 2 , wherein the capacitor consists of a series-connected set of capacitors. 
     
     
       6. An electric power conversion apparatus as recited in  claim 3 , wherein the capacitor consists of a series-connected set of capacitors. 
     
     
       7. An electric power conversion apparatus as recited in  claim 4 , wherein the inverter outputs three-phase ac by selecting a high-potential, a low-potential or a middle-potential, the high-potential represented at one end of the series-connected capacitors, the low-potential represented at the other end of the series-connected set of the capacitors, and the middle-potential represented at a connection point between the capacitors. 
     
     
       8. An electric power conversion apparatus as recited in  claim 5 , wherein the inverter outputs three-phase ac by selecting a high-potential, a low-potential or a middle-potential, the high-potential represented at one end of the series-connected capacitors, the low-potential represented at the other end of the series-connected set of the capacitors, and the middle-potential represented at a connection point between the capacitors. 
     
     
       9. An electric power conversion apparatus as recited in  claim 6 , wherein the inverter outputs three-phase ac by selecting a high-potential, a low-potential or a middle-potential, the high-potential represented at one end of the series-connected capacitors, the low-potential represented at the other end of the series-connected set of the capacitors, and the middle-potential represented at a connection point between the capacitors. 
     
     
       10. An electric power conversion apparatus as recited in  claim 7 , wherein the frequency fixing unit fixes timings of transition points for each phase where the phase voltage changes between the high-potential or the low-potential and the middle-potential, in the region where the high-potential and the low-potential are adjacent to each other with the middle-potential interposed therebetween, to predetermined values in relation to a carrier-wave or a signal-wave phase within a cycle, so as to fix to the command value a fundamental-wave frequency of the each-phase ac voltage outputted from the inverter. 
     
     
       11. An electric power conversion apparatus as recited in  claim 8 , wherein the frequency fixing unit fixes timings of transition points for each phase where the phase voltage changes between the high-potential or the low-potential and the middle-potential, in the region where the high-potential and the low-potential are adjacent to each other with the middle-potential interposed therebetwen, to predetermined values in relation to a carrier-wave or a signal-wave phase within a cycle, so as to fix to the command value a fundamental-wave frequency of the each-phase ac voltage outputted from the inverter. 
     
     
       12. An electric power conversion apparatus as recited in  claim 9 , wherein the frequency fixing unit fixes timings of transition points for each phase where the phase voltage changes between the high-potential or the low-potential and the middle-potential, in the region where the high-potential and the low-potential are adjacent to each other with the middle-potential interposed therebetwen, to predetermined values in relation to a carrier-wave or a signal-wave phase within a cycle, so as to fix to the command value a fundamental-wave frequency of the each-phase ac voltage outputted from the inverter. 
     
     
       13. An electric power conversion apparatus as recited in  claim 1 , wherein the heatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included. 
     
     
       14. An electric power conversion apparatus as recited in  claim 2 , wherein the beatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included. 
     
     
       15. An electric power conversion apparatus as recited in  claim 3 , wherein the beatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included. 
     
     
       16. An electric power conversion apparatus as recited in  claim 4 , wherein the beatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included. 
     
     
       17. An electric power conversion apparatus as recited in  claim 7 , wherein the beatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included. 
     
     
       18. An electric power conversion apparatus as recited in  claim 10 , wherein the beatless control unit and the frequency fixing unit are operated in a predetermined frequency range in which a frequency capacitor-voltage fluctuation generated according to the rectification by the converter is included.